This invention relates generally to mass spectrometry and particularly to a concave electrode ion pipe for transferring ions over long distances and between vacuum stages without significant ion loss.
Mass spectrometers have emerged as an important tool for analysis of biochemical samples, pesticides and organic compounds. They are highly sensitive instruments that have the capability of separating molecular ions according to a mass to charge ratio (m/z). A simple mass spectrometer includes three important components; the ionization source, mass filter and ion detector. Analytes may be introduced into the ionization source through a gas chromatograph, HPLC column or solid probe. In addition, since the ionization source, mass filter and ion detector are separated spatially, it becomes important to be able to move ions from place to place and chamber to chamber effectively and efficiently without loss of ions. In addition, it is quite often necessary to transfer ions between vacuum stages without significant ion loss.
Atmospheric pressure ion sources (API) including electrospray or nebulization assisted electrospray, atmospheric pressure chemical ionization (APCI), atomospheric pressure photo ionization (APPI), atmospheric pressure matrix-assisted laser desorption (AP MALDI) and inductively coupled plasma (ICP) have become increasingly popular and important for generating ions at atmospheric pressure in mass analysis. Various vacuum systems and associated electrostatic lenses have been extensively employed in these systems. Where there are several pumping stages maintained at 10xe2x88x921 to 10xe2x88x928 Torr, multipole ion guides have been employed. The use of multipole ion guides has been particularly important in transporting ions in vacuum or from one vacuum stage to another vacuum stage. A multipole ion guide can be designed to begin in one vacuum stage and extend contiguously through one or more additional vacuum stages of a multiple pumping stage system. In most cases when background pressure is high enough, the ions will scatter. The purpose then of the multipole ion guide is to prevent dispersions due to scattering. Ordinarily, significant loss of ions may occur when multiple stages are employed and ions must be moved from stage to stage. High ion transmission efficiency can be achieved by multiple vacuum pumping stages using multipole ion guides that have been configured to connect or extend between one or more vacuum stages. In practice, RF voltage is applied to the rods of a multipole guide, adjacent rods differing in phase by 180 degrees. The resulting electric field within the guide prevents ions from drifting too far from the axis of the guide, despite collisions of the ions with the background gas, and transmission of the ions is thereby enhanced. These ion guides, therefore, are effective in improving the performance of mass spectrometer systems by delivering more ions to the mass filter (analyzer). These ion guides, therefore, are effective in improving the performance of mass spectrometers. Examples of the types of mass spectrometer systems in which ion guides can be used include Time-of-Flight, Ion trap, FT-ICR, quadrupole, hybrid quadrupole/Time-of-Flight, orthogonal acceleration Time-of-Flight and magnetic sector. Ion sources that have been used for the various spectrometers incorporating ion guides include, for example, electrospray, atmospheric pressure chemical ionization, gas discharge, plasma and other sources that are known and used in the art.
In their simplest forms, RF multipole ion guides for ion transport in mass spectrometers are best illustrated in U.S. Pat. No. 4,963,736. These ion guides provide for transport of ions between vacuum stages or chambers. However, such ion guides may suffer from the disadvantage that many of the ions will not be transported, i.e. will contact the ion guide walls, and will fail to reach the exit end of the ion guide. Many ions will become xe2x80x9cstalled outxe2x80x9d if excessive background pressure is present. This is best exemplified in U.S. Pat. No. 5,847,386 that shows the effects on the ion diffusion or transport caused by the excessive background pressure in the ion guide. A high background pressure may be desirable for collisional focusing, but if the pressure is too high then the ions will undergo enough collisions with neutral atoms that they will no longer have significant axial kinetic energy to make it through the device in a practical time frame.
A number of techniques, designs and devices have been developed for avoiding this xe2x80x9cstall-outxe2x80x9d problem by re-accelerating the ions by application of an axial field. Efforts have been made to step DC potentials in ion guides to improve performance. This is accomplished by having a DC offset with each subsequent guide section such that the ions are reaccelerated by the field created in each transition region between the sections.
Mass spectrometers with these ion guides operated at high background pressure may not sufficiently limit the flow of gas to the mass filter. This can cause problems that lower overall performance in the instruments.
In a mass spectrometer system, it is necessary to limit the flow of gas to the mass filter. This is normally accomplished by one or more stages of flow restriction and pumping. Unfortunately, significant flow restriction limits the ion transmission efficiency that can be achieved; and insufficient flow restriction adds additional pumping stages or larger pumps that contribute to increased system cost and complexity.
It is, therefore, an object of the invention to provide an improved apparatus and method to serve as an ion pipe that can limit flow conductance and allow for the elimination of vacuum stages or use of lower speed pumps.
Another object of the invention is to provide a novel apparatus that will provide the ability to capture, focus and transport ions over a long distance without significant loss of ions and with significant gas flow reduction.
Another object of the invention is to provide a concave, segmented ion pipe for transporting ions over long distances that prevents ions from xe2x80x9cstalling-outxe2x80x9d between various instrument components of a mass spectrometer.
The invention includes a concave electrode ion pipe for delivering ions between vacuum stages. The concave electrode ion pipe includes a conduit having an axial bore that may connect at least two vacuum stages. The axial bore of the ion pipe defines a concave wall wherein the gas flows between the vacuum stages and the axial bore restricts the flow of gas. The concave wall is circumferentially segmented into electrodes to which are applied RF voltages alternating in phase between adjacent electrodes. The ion pipe may also be axially segmented in design and has the ability to carry ions over a long distance without substantial ion loss. The concave design of the pipe restricts the gas flow and allows for elimination of vacuum stages or application of lower speed pumps.